1. Field of the Invention
This invention relates to a method of sensing the rms value of an electrical waveform and in particular but not exclusively to a method of controlling the rms voltage in a load circuit supplied with a phase chopped sinusoidal voltage waveform.
2. Description of Related Art
The precise measurement of rms (root mean square) values of non-sinusoidal waveforms has hitherto required circuitry which is relatively complex and expensive. A typical analogue circuit computes the rms value by squaring the instantaneous value of input signal, dividing this result by the previously computed rms value and averaging the result to provide an updated rms value. The required linear circuits for the squaring and division calculations result in considerable complexity and expense if satisfactory results are to be produced.
A common circuit requirement is to provide in a load circuit a constant rms supply voltage by regulating a mains supply using a phase chopping device, the phase firing angle at which the sinusoidal mains voltage is chopped being adjusted to regulate the rms power in the load circuit in order to compensate for fluctuations in the amplitude of the mains supply voltage and fluctuations in the load impedance. The present invention seeks to provide a relatively simple method of sensing the rms value of the waveform in such circuits where the shape of the waveform is known to be that of a phase chopped sinusoidal waveform.